1. Field of the Invention
This invention pertains in general to the field of polymerizable compositions which are curable by various energy sources and in particular to apparatus and methods for determining gel time, initiator half-lives, and oxygen concentrations for prepolymer fluids commonly known as monomers.
2. Description of the Prior Art
In general, a class of materials have been developed through organic chemistry, known as monomers which exist as fluid at ambient temperatures but when cured, are changed into a hard, solid substance known as polymer. Two major characteristics of polymers are that they form a thin and hard coating and that they tenaciously adhere to a substrate surface. Again, in general, certain monomers may be combined with a cure preventing inhibitor, such as oxygen and a chemical reaction initiator such as borate. There are other inhibitors and many other initiators. The oxygen inhibitor maintains the monomer in a fluid state; accordingly, upon the depletion of oxygen, the initiator converts the monomer into a polymer. The initiator, among other things, determines the particular type of energy which may be used to cure the monomer. For example, monomers may be cured by heat, white light, ultraviolet light, radiation electron beam, and laser energy sources. Borate is generally light curable and is also sensitive to heat but to a lesser degree.
The tenacious adhering and hardness properties of polymers have relatively recently found commercial acceptance in the field of thin coatings applied onto a base material. Thus, in the past, the surface coating of such items as beverage cans and printing of paper boxes and magazines were previously accomplished by the use of paints or inks which dried in either heated or ambient air conditions. These traditional coatings have been substantially replaced by polymerizable coatings which are curable by one or more of the above-mentioned energy sources. For example, industry now routinely uses curable lacquer, curable acrylic, curable ink, curable wood finishes, and even curable adhesives made from monomers in place of the previous traditionally dried compositions. In these applications, a polymerizable composition comprising a monomer and the basic product is mixed with an initiator which, during an induction period of exposure to an energy source, removes oxygen within the composition and thereafter polymerizes the coating into a very hard, long-lasting thin film. The initiator within the polymerizable composition results in the rapid curing of the composition when acted upon by means to which the initiator is sensitive.
This recent use of fast-curing compositions for thin coating applications has resulted in numerous advantages. For example, extremely fast production line speeds are achieved. More flexibility and less waste are achieved. In general, the technique is non-polluting, which eliminates discharge problems, expensive air cleaning, and results in better plant conditions. Multicolor printing is made much easier and cheaper. The finished product achieves a much higher quality, a better appearance and finish, and performs better. Understandably, the coating of products by polymerization has achieved widespread acceptance by many industries in a myriad of different fields.
The use of monomers or polymeric compositions is not limited to the field of thin coatings. Polymerization is, for example, used to manufacture rubber tires. In this field, a thermal initiator is used such that when the liquid material fills the tire mold, the application of heat turns the liquid into solid rubber in accordance with the gel time of the polymerizable composition.
In the class of monomers which are oxygen inhibited, the oxygen determined-induction exposure time sets the lower limit for curing time since this is the exposure time required by an energy source (to consume the oxygen within the polymerizable composition) before any polymer chemistry can begin. That is, the induction time sets the minimum exposure time required in a curing operation. As such, oxygen is a natural inhibitor to free radical polymerization chemistry and represents expended energy before curing can take place. Oxygen is likewise a principal stabilizer in free radical systems (polymerizable compositions) and is necessary for storage, processability, and shipability of the same.
Over the years, different techniques have been developed in order to determine the gel time (the time required to initiate cure in a monomer) for variously-initiated polymerizable compositions under reproducible conditions. Among these, one prior art method involves the use of an infra-red energy. This technique utilizes the change in absorbance corresponding to the disappearance of the carbon-carbon double bonds on photoinitiated polymerization and deals more with the state of cure rather than the gel point beginning of cure. Another prior art method uses the contraction of the photocuring formulation. In this technique, an interface between water and the photoinitiated formulation forms part of a capacitor. The change in capacitance of the arrangement is directly proportional to the position of the air-water interface. Thus, measuring the time involved with the change in capacitance, determines the curing time of the composition.
Still other techniques for determining the gel time or curing time of polymerizable compositions comprise a differential scanning calorimetry method, a pulsed NMR method, and a photoacoustic spectroscopy method.
A prior art commercially available system for the study of photoinitiated polymerizations comprises the Perkin-Elmer DPA7 double beam photo-calorimetric accessory. This apparatus permits the laboratory investigation of the effects of ultraviolet light on a wide variety of materials. Photo-calorimetry provides a means for monitoring of the curing process and the evaluation of the effects of temperature, different ultraviolet wavelengths, influence of atmosphere, and the varying amounts and types of photoinitiators. This apparatus uses a differential scanning calorie meter measuring the amount of energy absorbed or released from a sample with the temperature being precisely controlled. Thus, the Perkin-Elmer apparatus measures energy changes resulting from temperature-induced reactions.
In general, the above-described apparatus and methods are too sophisticated or too complex to be used for production-related control functions, and, in general, are not so used. One such production-related use is to ensure the consistency of reactivity (gel time) of supplied polymerizable formulations. That is, whether the supplied raw materials will perform within a given set of predetermined specifications as compared to the polymerizable composition used to establish a pilot manufacturing sequence.
Repeatability of cure times is an important manufacturing parameter. It assures that a manufacturing sequence following the application of a surface coating does not begin too soon or too late. If it begins too soon, the polymer will not have sufficiently cured so that the finished coating is not acceptable. If the next manufacturing sequence occurs too late, the result is a loss of manufacturing time which decreases overall production and decreases efficiency.
Accordingly, a primary object of the present invention is to provide methods and apparatus for measuring the incipient gel time of a polymerizable composition which is repeatable and simple and directly associated with production line consistency.
Another object of the present invention is to provide simple methods and apparatus which can determine the gel time of polymerizable compositions regardless of the energy means or type, such as monochromatic light, white light, uv, heat, electron beam, catalytic, etc.
Another object of the present invention is to provide methods and apparatus for determining the gel time of polymerizable compositions which are simple, inexpensive, reliable, and consistently determine the gel time by a method which readily discerns the gel time.
Since inhibiting polymerizable compositions with oxygen comprises the most widely used inhibitor, direct knowledge of the rate of depletion of oxygen is important to be known. Similarly, direct knowledge of oxygen diffusion and oxygen stability are also important to be known.
Accordingly, another object of the present invention is to provide apparatus and methods which can determine the relative concentration of dissolved oxygen within oxygen-inhibited, polymerizable compositions.
Another object of the present invention is to provide methods and apparatus that can be used to determine oxygen diffusivity in oxygen-inhibited polymerizable compositions.
Yet another object of the present invention is to provide methods and apparatus that can be used to determine oxygen depletion rates in oxygen-inhibited polymerizable compositions.
Still another object of the present invention is to provide apparatus and methods that can be used to determine thermal initiator reactivities (half-lives) in thermally-initiated polymerizable compositions whereby the initiator exists not independently of but rather within the polymerizable composition.
The above-stated objects as well as other objects which, although not specifically stated, but are intended to be included within the scope of the present invention, are accomplished by the present invention and will become apparent from the hereinafter set forth Detailed Description of the Invention, Drawings, and the claims appended herewith.